Since a lithium secondary battery (lithium ion secondary battery) has a small size and large capacity, it is widely utilized for applications such as a portable electric device and a personal computer. However, while a rapid development of portable electric devices or the use for electric vehicles have been realized in recent years, further improvement in energy density has been an important technical subject.
There are several methods to increase energy density of a lithium secondary battery. Among them, it is effective to increase an operation potential of the battery. In a lithium secondary battery using conventional lithium cobalt oxide or lithium manganate as a positive electrode active material, the operation potential of each case is 4 V class (average operation potential=3.6 to 3.8 V: versus lithium potential). In these positive electrode active materials, the operation voltage is determined by oxidation-reduction reaction of a Co ion or Mn ion (Co3+↔Co4+ or Mn3+↔Mn4+).
In addition, it is known that an operation potential of 5 V class can be achieved by, for example, using a spinel compound in which Mn of lithium manganate is replaced with Ni or Co, Fe, Cu, Cr and others, as an active material. Specifically, as in Patent Document 1, it is known that a spinel compound such as LiNi0.5Mn1.5O4 shows a potential plateau in the area of 4.5 V or more. In such a spinel compound, Mn is present in the quadrivalent state, and the operation potential is determined by oxidation-reduction of Ni2+↔Ni4+ instead of oxidation-reduction of Mn3+↔Mn4+.
For example, LiNi0.5Mn1.5O4 has a capacity of 130 mAh/g or more, and the average operation voltage is 4.6 V or more versus lithium metal. Although its capacity is smaller than that of LiCoO2, energy density of the battery is higher than that of LiCoO2. Furthermore, a spinel type lithium manganese oxide has a three-dimensional lithium diffusion path and also has advantages such as excellent thermodynamic stability and easy synthesis. For these reasons, LiNi0.5Mn1.5O4 holds promise as a future positive electrode material.
As for an electrolyte solution used in a lithium secondary battery, the examples described in the following documents are proposed.
Patent Document 2 discloses an electrolyte solution that contains a phosphate ester and a compound having a sulfone structure. According to this document, it is disclosed that swelling deformation of the battery exterior during high temperature storage can be prevented in a lithium secondary battery using a 4 V class electrode.
Patent Document 3 discloses a nonaqueous electrolyte solution for battery, which contains: an unsaturated phosphate ester compound as (A) component; at least one compound selected from the group consisting of a sulfite ester compound, a sulfonate ester compound, an imide salt compound of an alkali metal, a fluorosilane compound, an organic disilane compound and an organic disiloxane compound as (B) component; an organic solvent as (C) component; and an electrolyte salt as (D) component. According to this document, it is disclosed that less inner resistance and high electric capacity can be maintained during long-term use in a nonaqueous electrolyte secondary battery which has a negative electrode manufactured comprising a highly crystalline carbon material such as graphite as an active material, and a high molecular weight carboxylic acid compound as a binder.
In Patent Document 4, a secondary battery that has an electrolyte solution containing a fluorine-containing phosphate ester is disclosed.
Patent Document 5 discloses an electrolyte solution containing, a carbonic acid ester having a halogen, at least one compound from dicarbonates represented by a specific formula, dicarboxylic acid esters and disulfonic acid esters, and phosphoric acid esters. The same document discloses that the solvent may further contain a cyclic carbonate ester having an unsaturated bond, a sultone, an acid anhydride and the like.
Patent Document 6 discloses a nonaqueous electrolyte solution containing a monofluorophosphate salt and/or a difluorophosphate salt. This document furthermore discloses that good battery characteristics can be maintained if at least one compound A is added to the electrolyte solution, and discloses phosphoric acid esters and acid anhydrides, as compound A.